There are a variety of methods for transmitting information via a broadband Integrated Services Digital Network (B-ISDN), using a variety of protocols related to Asynchronous Transport Mode (ATM), frame relay mode, ISDN and SS#7 modes of transmission. The ATM mode, as the exemplary preferred embodiment, was originally investigated by a group called the International Telephone and Telegraph Consultative Committee (CCITT). The group, currently called the International Telecommunication Union—Telecommunications Standards Sector (ITU-TSS), investigated a new form of ISDN that would have the flexibility to accommodate a large number of channels and the ability to transfer large amounts of data at a very fast rate. At the end of the study, the committee decided to adopt ATM as the target transfer mode for the B-ISDN. The ITU-TSS is currently defining the wide area network (WAN) standards for ATM.
ATM is a transfer mode that sends 53 octet-sized packets of information across a network from one communication device to another. The 53 octets are assembled as a “cell”, which comprises 48 octets of data information, referred to as the “payload”, and 5 octets of “header” information (including the routing information). The header and data information must be organized into cells so that when the cells are filled, they can be sent when an open slot of 53 octets becomes available.
There are two forms of headers that are specified in the CCITT Recommendation I.361. Each form is 5 octets long. There also are two different ATM network connections, each one having a different type of header. One connection form is the user-network interface (UNI), which is used between the user installation and the first ATM exchange and also within the user's own network. The other form of connection is the network-node interface (NNI) which is used between the ATM exchanges in the trunk network. The header format for the UNI consists of the following fields:                Generic flow control (GFC) field of 4 bits. It can provide flow control information towards the network from an ATM endpoint.        Routing field of 24 bits. Eight of the bits are VPI (virtual path identifiers) and 16 bits are VCI (virtual channel identifier). Empty cells with both the VCI and VPI set to zero indicates that the cell is unassigned.        Payload type (PT) field of 3 bits. This field is used to provide information on whether the cell payload contains user information or network information. This field is used by the network to intercept any important network information.        Cell loss priority (CLP) field containing 1 bit. This field may be set by the user or service provider to indicate lower priority cells. If the bit is set to 1 the cell is at a risk of being discarded by the network during busy times.        Header error control (HEC) field of 8 bits. This field is processed by the physical layer to detect errors in the header. The code used for this field is capable of either single-bit error-correction or multiple-bit error-detection.        
As seen in FIGS. 1A and 1B, the header format for the NNI is the same as the header information of the UNI except that there is no GFC, and the VPI of the routing field is 12 bits instead of 8 bits.
Error detection occurs only within the header message. No error detection of the data occurs within the information portion of the cell. The receiving endpoint determines whether the data can be corrected or whether it must be discarded. When a link or node becomes busy, an ATM network must discard cells until the problem is resolved. The first cells to be discarded are the cells that have a CLP bit in the header set to a “1”. Since connection endpoints are not notified when a cell is discarded, higher layers of protocols are needed to detect and recover from errors.
A cell is sent along a channel called a Virtual Channel Connection (VCC). A VCC consists of a series of links that establish a unidirectional connection through the network that allows the flow of information from one endpoint to another endpoint. Cells on a VCC always follow the same path through the network. Therefore, each cell arrives at its destination in the same order in which it was transmitted. VCCs can be unidirectional or may occur in pairs, thus making the connection bi-directional. VCCs can be within a Virtual Path Connection (VPC), meaning a group of virtual channels that are associated together, so as to be sent as a large trunk for a part of network. The VCCs are multiplexed and demultiplexed at appropriate network nodes in the network. Each VCC and VPC have specially assigned numbers called Virtual Channel Identifiers (VCI) and Virtual Path Identifiers (VPI), respectively. These numbers help the system determine the direction in which the cells belonging to the connection should be sent and which applications should be associated with the connection.
Although ATM-based transmission, switching, and network technology has been employed in broadband integrated services digital networks (B-ISDN) which rely on fiber optics, there are numerous difficulties associated with implementing ATM based technology in a wireless communication network. These difficulties include the fact that ATM-based networks and switches rely on a number of high speed interfaces. These high-speed standard interfaces include OC-3 (155 Mbit/s), OC-12 (622 Mbit/s) and DS3 (45 Mbit/s). However, a few ATM based networks and switches support lower speed interfaces, such as T1 (1.544 Mbit/s) and the programmable rate RS-449 interface.
As a consequence, there are only a few interfaces which can support the comparatively low transmission rates (less than 1 Mbit/s to a 8 Mbit/s) used in wireless communication. Although commercial satellite and wireless modems support these low transmission rates using an RS-449 programmable rate interface, it is difficult to implement ATM based technology in a wireless environment using conventional interfaces, such as the satellite environment seen in FIG. 2, because most ATM traffic is transmitted over high rate data interfaces. FIG. 2 illustrates the interconnection by a satellite relay between multiple terminals (Terminal 1 and Terminal 2, merely by way of example), using a time division multiple access (TDMA) network wherein bursts of information are sent to the satellite in a time divided manner for assembly and distribution to the terminals in a frame format.
Another difficulty associated with implementing ATM based technology in a wireless communication network has to do with the fact that ATM based protocols rely on extremely low bit error ratios which are typical of fiber optics based networks. By way of example, ATM protocols assume that the transmission medium has very low Bit Error Ratios (BER) (10−12) and that bit errors occur randomly.
In contrast, the bit error ratios associated with wireless communication are much higher (on the order of 10−3 to 10−8) and tend to fluctuate in accordance with atmospheric conditions. In addition, the errors associated with wireless communication tend to occur in longer bursts. Thus, a robust error correction scheme must be employed in a wireless network in which ATM based technology is to be implemented.
In addition to the difficulties discussed above, there is another significant constraint placed on wireless communication networks which is not imposed on terrestrial based fiber optics networks. This constraint has to do with the fact that the cost of bandwidth in a wireless network is much higher than for fiber optics networks. As a consequence of having been traditionally implemented in fiber optics networks, ATM based technology is not particularly efficient in its use of transmission bandwidth. Therefore, if ATM-based technology is to be implemented in wireless networks, it must achieve a more efficient use of bandwidth.
Networking protocols have evolved over the years. Protocols have evolved from supporting the low rate X.25 kind of services to carrying high rate ATM traffic. Therefore, today's networks are required to support several different protocols like X.25, TCP/IP, Frame Relay, ISDN, SS7, etc. Protocols such as Frame Relay, IP and X.25 have variable size traffic. ATM on the other hand has fixed size cells. Therefore a network supporting multiple protocols must be capable of transporting variable size data.
Transmission links themselves limit the maximum packet size that can be transmitted over the link. This requires a variable length packet to be split into smaller segments before transmission and be put together at the destination before delivering to the user. The process of splitting the packet is called segmentation. The process of reconstructing the original packet is called reassembly.
Often nodes in a network are interconnected using multiple transmission links. This is done sometimes to get additional bandwidth, sometimes because the network topology demands it, or for redundancy. When packets are transmitted over these links, they may appear out of order at the destination node. Users expect that packets be delivered back to them in the order in which they were sent to the network. Therefore, packets need to be resequenced before they are delivered to the user. This process is also commonly referred to as “inverse multiplexing”. The terms “resequencing” and “inverse multiplexing” are used interchangeably herein.
Networks have also evolved in complexity. Older networks traditionally used static bandwidth management schemes. Today's networks do “bandwidth-on-demand”, i.e. bandwidth is continuously changing while the network is carrying traffic. Therefore, the solutions developed should work in a bandwidth-on-demand environment.
The problems of segmentation, reassembly and resequencing have been addressed before. However, existing solutions do not meet the needs of a bandwidth-on-demand satellite network. Some of the current approaches have relatively large segment headers. Bandwidth is a precious resource in a satellite network. If these solutions were to be applied to a satellite network, they would waste a lot of bandwidth. This is especially important if the network must carry ATM traffic, since the payload size per cell is fairly small (ATM cells have a 5 byte header and 48 bytes of data). Also, the current algorithms, like the IP reassembly algorithm, are more complicated and have a higher processing power requirement. This makes it harder to implement the algorithm in software for intermediate data rates (e.g. 8 Mbits/sec). Special hardware is required, which raises the cost of the product.
In bandwidth-on-demand networks, the bandwidth to different destinations from each node is continuously changing. The above algorithms for segmentation, reassembly and resequencing must be capable of adapting to the varying bandwidth.
Some cell- or packet-based protocols, such as Frame Relay and ATM, are connection oriented while others, like IP are datagram oriented. Any solutions developed should be able to work with both connection- and datagram-oriented protocols. This also makes it easier to handle network management traffic.
Low rate modems are simpler and cheaper to build. They also impose less stringent power requirements on the system. Therefore, it is desirable that each terminal be equipped with a low rate modem, that terminals be stacked up, and that every terminal in the stack be allowed to use spare capacity on other terminals. This is a simpler and less expensive system model as opposed to a single high rate modem. Segments may then have to travel through intermediate terminals on the path from the source to the destination. The algorithms should be capable of supporting this model.
Two algorithms that have been used for segmentation and reassembly in the past are the IP and the ATM AAL-5 s algorithms. However, each of these algorithms has deficiencies that are overcome by the present invention.
The IP segmentation algorithm inserts a packet number and the byte offset of the segment relative to the start of the packet in every segment. The use of a byte offset is required because IP has to deal with different protocol data unit (PDU) sizes across different transmission links in the network. IP packet headers are fairly large, which results in a substantial amount of wasted bandwidth. Also, the IP reassembly algorithm has to keep a list of previously received segments of each packer, typically in a predetermined location. Therefore, on receiving a new segment, a search must be performed in order to find the proper location of the new segment in the list.
The ATM AAL-5 algorithm works on a per-virtual circuit (VC) basis. Therefore, it cannot handle datagram traffic without first creating special VCs to carry such traffic.
Traditionally inverse multiplexing has been circuit-based, used primarily to get more bandwidth between two nodes using multiple fixed rate trunks and to compensate for variable delays encountered in the network. The traditional model for inverse multiplexing is shown in FIG. 3. In this model the source network nodes 10 and the destination network nodes 20 are connected to a network 50 via special hardware units called an inverse multiplexer 30. Each node, in fact, will have both source and destination components, as shown. The inverse multiplexers 30 are also interconnected using multiple low or intermediate rate trunks 40. The assembly provides a high rate pipe between the two network nodes.
In the case of a bandwidth-on-demand satellite network, the transmission capacity to different destinations increases and reduces dynamically. However, the traditional inverse multiplexing model does not permit dynamic changes in bandwidth or, in particular, the use of spare capacity on other co-located terminals for sending segments through them.
More recently, packet-based inverse multiplexing schemes, such as the one used by the Multilink Point-to-Point (MLPTP) have been proposed. However, in that method, each segment needs a sequence number, a requirement that increases header size and reduces efficiency.
Accordingly, it is an object of the present invention to provide a method and apparatus for the segmentation, reassembly and inverse multiplexing of packets and cells over satellite/wireless links.
It is a further object of the present invention to provide a low transmission rate interface comprising a method and apparatus for the segmentation, reassembly and inverse multiplexing of packets and cells.
It is yet another object of the present invention to provide an interface that supports a plurality of cell- and packet-based protocols with both fixed and variable sized traffic.
It is another object of the present invention to provide a low transmission rate interface comprising a method and apparatus for the segmentation, reassembly and inverse multiplexing of packets and cells using protocols that are either datagram or connection oriented.